1. Field of the Invention
This invention relates to the field of hydraulic systems, more particularly it pertains to control of valves used in such systems to operate an automatic transmission for motor vehicles.
2. Description of the Prior Art
A gear ratio change in a conventional automatic transmission is accomplished by applying or releasing a friction element (a brake or clutch) that changes the speed and torque relationship by means of planetary gearsets. Friction elements are hydraulically controlled by pressure regulator valves and flow regulator (shift control) valves. Pressure regulator valves control hydraulic pressure, which determines friction element torque capacity. Flow regulator valves, which control a flow rate of pressurized oil into a friction element, determines the time rate of friction element torque capacity change.
A continuously variable transmission (CVT) uses a special variator to change the speed and torque relationship continuously. Variators are hydraulically controlled by pressure regulator valves and flow regulator valves. Pressure regulator valves control hydraulic pressure, which determines the variator torque capacity. Flow regulator valves, which control the flow rate of fluid into variator pistons, determines the rate of speed ratio change. In both conventional automatic transmissions and CVTs, precise control of the valve motion is vital to providing and maintaining good shift quality and durability.
Conventional automatic transmission systems do not monitor motion of pressure and flow regulator valves. oscillatory motion of pressure regulator valves can cause undesirable hydraulic pressure oscillation, which can degrade shift quality. Also, it can potentially emit audible noise. Conventional flow regulator valves may include a fixed flow control geometry called "notches". This passive method may not perform optimally in all situations. Unit-to-unit variations and component degradation of pressure and flow regulator valves over the life of the transmission may cause shift quality degradation with the conventional approach.